1. Field of the Invention
The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine. More particularly, the present invention pertains to an air-fuel ratio control apparatus provided with an air-fuel ratio feedback control function and a purge control function.
2. Description of the Related Art
The evaporated fuel, which is generated from a fuel tank and the like, has been adsorbed by activated charcoal and purged into an air intake system.
Further, there are internal combustion engines which carry out air-fuel ratio feedback control so that the air-fuel ratio of a gas mixture in a fuel injector is made to a stoichiometric value.
In these internal combustion engines, when the evaporated fuel is not purged, an air-fuel ratio feedback correcting coefficient varies around, for example, 1.0. However, when a purging operation is started, the air-fuel ratio feedback correcting coefficient is set to a smaller value because a fuel injection amount must be reduced by the amount of the evaporated fuel having been purged.
The deviation of the air-fuel ratio feedback correcting coefficient from a reference value during purging operation takes various values, depending upon the operating state of the internal combustion engine, that is, according to the ratio of the amount of intake air to the amount of purge air. The air-fuel ratio feedback correcting coefficient is set so as to relatively slowly change with a given integration constant to avoid an abrupt change in the air-fuel ratio. When the purge rate is changed by a transient engine operation and the like while the purging operation is being carried out, it takes some time for the purge rate to settle to a certain value after the change of the purge rate from the value before the change thereof, so that the air-fuel ratio cannot be maintained to the stoichiometric value during that time.
To cope with the above problem, Japanese Patent Laid-Open No. 5-52139 proposes the following apparatus. That is, an internal combustion engine disclosed therein includes a first injecting amount correction means for correcting a fuel injection amount by an air-fuel ratio feedback correcting coefficient, a purge air density calculation means for calculating a purge air density per a unit target purge rate based on the deviation of the air-fuel ratio feedback correcting coefficient, which would be caused when purging operation is carried out, and a second injecting amount correction means for reducing a fuel amount based on the product of the purge air density and the purge rate when the purging operation is carried out. In the internal combustion engine, a maximum purge rate, which is the ratio of the purge amount to the intake air amount with a purge control valve being fully opened, is previously stored, the duty factor or ratio of the purge control valve is set to the ration of a target purge rate to a maximum purge rate, and when the purging operation is started, a target duty ratio is gradually increased. When the air-fuel ratio feedback correcting coefficient is equal to or less than a prescribed value and an air-fuel ratio is on a rich side, a purge air density coefficient is increased stepwise by each given value. In addition, the deviation of the air-fuel ratio feedback correcting coefficient is reflected on the purge air density coefficient at a prescribed rate at each 15 seconds after the start of the purging operation. With this operation, the air-fuel ratio feedback correcting coefficient is forcibly caused to approach 1.0. As described above, the duty ratio of the purge control valve is controlled so that the purge rate is made constant regardless of the operating state of the engine. In addition, even if the purge rate is changed, the deviation of the air-fuel ratio during transient engine operation is prevented by correcting the injection amount by the product of the purge rate and the purge air density.
However, even if the duty ratio of the purge control valve is controlled so that the purge rate is made constant, and even if the injection amount is controlled by the product of the purge rate and the purge air density, a certain period of time is necessary until the air-fuel ratio feedback correcting rate is made to 1.0. Accordingly, there arises a problem that the air-fuel ratio cannot be maintained to the stoichiometric value in the state in which the purge air density has not yet completely been calculated, that is, when a purge-cut state is shifted to a purged state, or when the state, in which a purge rate of several percentages is secured with a medium load, is shifted to the state, in which the purge rate is approximately zero with a high load, or vice versa.
To solve the above problem, Japanese Patent Laid-Open No. 8-261038, which was previously filed by the applicant, proposes an air-fuel ratio control apparatus for an internal combustion engine which can control the air-fuel rate of a mixture to be introduced into the internal combustion engine to a target value with excellent accuracy at all times. That is, the air-fuel ratio control apparatus comprises a purge rate calculation means for calculating a purge rate of purge air from the purge amount thereof, which is calculated by a purge amount calculation means and from the operating state of the engine, which is detected by an operating state detection means, a purge air density calculation means for calculating a purge air density from the purge rate and an air-fuel ratio feedback correcting coefficient, a purge air density correction means for calculating a purge air density correcting coefficient based on the purge rate and the purge air density, and a fuel injection amount calculation means for calculating a fuel injection amount which is supplied to the internal combustion engine based on the air-fuel ratio feedback correcting coefficient and the purge air density correcting coefficient. The air-fuel ratio control apparatus calculates the purge air density from a deviation of the air-fuel ratio feedback correcting coefficient and from the purge rate when a purging operation is carried out, calculates the purge air density correcting coefficient based on the purge air density and the purge rate, and calculates a fuel injection amount, which is supplied to the internal combustion engine, based on the air-fuel ratio feedback correcting coefficient and the purge air density correcting coefficient.
The air-fuel ratio control apparatus controls the air-fuel ratio feedback correcting coefficient to a target value by correcting the fuel injection amount according to the purge rate and the purge air density. Further, the air-fuel ratio control apparatus calculates a learned purge air density value by subjecting the purge air density, which is calculated by the purge air density calculating means, to filter processing. When the purge air density is calculated for the first time after the engine is started, the air-fuel ratio control apparatus uses the result of the calculation as the learned purge air density value without subjecting the purge air density to the filter processing. When the purge rate is equal to or less than a prescribed value, the air-fuel ratio control apparatus prohibits the update or renewal of the purge air density. Furthermore, after the purge air density has been calculated, the air-fuel ratio control apparatus makes the increasing rate of the purge amount, which is gradually increased after the start of the internal combustion engine, greater than the increasing rate thereof before the purge air density has been calculated.
However, the conventional air-fuel ratio control apparatus has the following disadvantages.
1) Although the purge air density is sufficiently learned, the opportunity for learning the air-fuel ratio feedback is not sufficiently secured. That is, the opportunity for learning the air-fuel ratio feedback is less than the opportunity for learning the purge air density, and both the opportunities are not equally provided.
2) During the time when the air-fuel ratio feedback is being learned, a sufficient flow rate of purge air cannot be secured.
3) When the density of purge air has not yet been learned, it is impossible to suppress variations in the air-fuel ratio due to introduction of purge air. In addition, when the purge air density has been learned, a sufficient flow rate of purge air cannot be secured.
4) Variations in the air-fuel ratio, which would be caused by a deviation between the learned purge air density value and an actual purge air density value during continued non-introduction of purge air, cannot be prevented.